1. Field of the Invention
The present invention relates to a variable gain amplifier, and more particularly, to a variable gain amplifier having stable input/output matching characteristics.
2. Description of the Related Art
In transmitting a signal using a wireless communication apparatus, in order to maintain the power of a signal output via an antenna constant, or to provide a signal received via the antenna with an appropriate gain according to the intensity of the received signal, the wireless communication apparatus needs a variable gain amplifier.
FIG. 1 is a circuit diagram of an exemplary wireless communication apparatus including a variable gain amplifier.
The operation of the wireless communication apparatus associated with the transmission of signals will now be described in detail with reference to FIG. 1.
Referring to FIG. 1, a baseband signal output from a baseband processor 190 is amplified by a baseband amplifier 140. The amplified baseband signal is mixed with an oscillation signal generated by an oscillator 180 by an up-mixer 130, thereby generating a radio frequency (RF) signal. Known communication systems convert a baseband signal into an intermediate frequency (IF) signal and then convert the IF signal into an RF signal instead of directly converting a baseband signal into an RF signal. The RF signal generated by the up-mixer 130 is amplified by a power amplifier (PA) 120, and the amplified RF signal is output via an antenna 110. The power amplifier 120 may comprise multiple-stage amplifiers to achieve a high gain while reducing distortion. For example, the wireless communication apparatus may include a pre-power amplifier and a power amplifier.
The operation of the wireless communication apparatus associated with the reception of signals will now be described in detail.
An RF signal is received via the antenna 110 and then is amplified by a low noise amplifier (LNA) 150. The amplified RF signal is converted into a baseband signal by a down-mixer 160, and the baseband signal is amplified by a baseband amplifier 170. Conventional communication systems convert an RF signal into an intermediate frequency (IF) signal and then convert the IF signal into a baseband signal instead of directly converting an RF signal into a baseband signal. The amplified baseband signal is transmitted to a baseband processor 190. The low noise amplifier 150, like the power amplifier 120, may comprise a plurality of amplifiers to achieve a sufficiently high gain.
A switch 115 prevents the RF signal output from the power amplifier 120 from being input to the low noise amplifier 150 and prevents the RF signal received via the antenna 110 from being transmitted to the power amplifier 120. A duplexer rather than the switch 115 may be used in a full-duplex communication system.
In short, a power amplifier (PA) or a low noise amplifier (LNA) used in a wireless communication apparatus must provide a sufficiently high gain. In this regard, Japanese Patent Publication No. 2003-243951 discloses a variable gain amplifier having means for varying impedance. In detail, the variable gain amplifier includes an impedance variation unit located between the collectors of a differential pair of bipolar transistors. However, the variable gain amplifier has a small gain adjustment range and shows unstable input/output matching characteristics especially when being used as a high frequency circuit.
FIG. 2 is a circuit diagram of an example of a conventional variable gain amplifier having a wide gain adjustment range.
Referring to FIG. 2, the variable gain amplifier has a differential cascode amplifier structure including a differential pair of cascode amplifiers, i.e., first and second cascode amplifiers, and also has an LC matching structure for input/output matching.
The first cascode amplifier includes a common source transistor 211 and a common gate transistor 221, and the second cascode amplifier includes a common source transistor 212 and a common gate transistor 222.
A bias voltage VG1 and a positive signal (+) input via a signal input terminal are applied to the gate of the common source transistor 211. The common source transistor 211 improves the frequency response characteristics of the first cascode amplifier. The positive signal (+) passing through the first cascode amplifier is output via an output terminal Out+. For input matching, a capacitor and an inductor are connected to the signal input terminal through which the positive signal (+) is received.
Likewise, the bias voltage VG1 and a negative signal (−) input via a signal input terminal are applied to the gate of the common gate transistor 222. The common gate transistor 222 improves the frequency response characteristics of the second cascode amplifier. The negative signal (−) is output from the second cascode amplifier via an output terminal Out−. For input matching, a capacitor and an inductor are connected to the signal input terminal through which the negative signal (−) is received.
A gain adjustment unit 231 includes two transistors that serve as switches and a resistor. The turning on or off of the two transistors is controlled by a gate voltage Vcont applied to the two transistors. Inductors 241 and 242 are used for output matching.
FIGS. 3A-3D illustrates the variations of the values of S-parameters S11, S21, and S22 indicating the input/output matching characteristics of the conventional variable gain amplifier shown in FIG. 2.
Specifically, FIGS. 3A and 3B illustrate the variations of the values of the S-parameters S11, S21, and S22 in a high gain mode, and FIGS. 3C and 3D illustrate the variations of the S-parameters S11, S21, and S22 in a low gain mode.
In a high gain mode, the S-parameter S11 indicating input matching characteristics has a value of −22.279 dB, and the S-parameter S22 indicating output matching characteristics has a value of −20.576 dB. Given that a variable gain whose input/output matching properties are lower than −15 dB is generally considered desirable, it appears that the conventional variable gain amplifier shown in FIG. 2 has desirable input/output matching characteristics in the high gain mode.
On the other hand, in the low gain mode, the S-parameter S11 has a value of −23.193 dB, and the S-parameter S22 has a value of −7.082 dB. Thus, the conventional variable gain amplifier shown in FIG. 2 is also considered desirable in the low gain mode in terms of input matching characteristics. However, the conventional variable gain amplifier shown in FIG. 2 is undesirable in the low gain mode in terms of output matching characteristics because the output impedance of the conventional variable gain amplifier shown in FIG. 2 varies depending on whether the gain adjustment unit 231 is turned on or off.
A variable gain amplifier optimized for a high gain mode is highly likely to end up having poor input/output matching characteristics in a low gain mode, while a variable gain amplifier optimized for the low gain mode is highly likely to end up having poor input/output matching characteristics in the high gain mode.